An electro-optic waveguide grating can modulate the intensity of the light transmitted through it, or reflected by it, based on an applied electric field. So an electro-optic waveguide grating can function either as an optical modulator whose modulating is affected by the electric field or as an electric field detector. If an electrical signal is purposefully generated for transmission purposes, then an electro-optic waveguide grating can be used to modulate the generated electrical signal onto an optical carrier signal. On the other hand, if an electric field needs to be sensed, then an electro-optic waveguide grating can be used to sense the electric field by modulating its temporal variation onto an optical carrier signal. So an electro-optic waveguide grating can thought of as either (or both) a sensor and a modulator.
The strength of the modulation produced by an electro-optic waveguide grating, i.e. the modulation depth of the modulated optical signal, increases as the overall length of the grating is increased. However, with a sufficiently long grating, it is possible that the light will not fully pass through the grating before the time-varying modulation signal has changed from producing a positive modulation (increased intensity of the light) to a negative modulation (decreased intensity of the light). Thus, the length of the grating sets a limit on the maximum frequency of the applied electric field (and in many applications the applied electric field is a radio frequency or RF field) that controls the modulation and conversely, the maximum frequency of the applied modulation-controlling signal sets a limit on the maximum length for a grating.
Although this application sets the length of each grating to be below that maximum length for a given modulation frequency, this application achieves stronger modulation than would be obtained for a single grating by passing the light through multiple gratings, with additional modulation of that light produced at each successive grating. By properly spacing these successive gratings, a high modulation frequency can be supported. However, the bandwidth of that modulation signal is limited. For example, assume that the center modulation frequency (carrier frequency) of an RF electric field is 10 GHz and that the RF signal modulated onto that RF electric field has a maximum frequency of 1 GHz. The frequency content (or the bandwidth) of the modulation waveform then extends from 9 to 11 GHz (assuming amplitude modulation—frequency modulation would likely result in a wider bandwidth).
In the prior art, optical modulators based on a single grating formed in electro-optic material have been described in articles by An, Cho and Matsuo (IEEE Journal of Quantum Electronics, vol. QE-13, no. 4, April 1977, pp. 206-208), by Cutolo et al. (Applied Physics Letters, vol. 71, no. 2, 14 Jul. 1997, pp. 199-201) and by Kim et al. (Electronics Letters, vol. 41, no. 18, 1 Sep. 2005). FIG. 1 shows an illustration of such a prior art modulator. All of these modulators use non-travelling-wave RF electrodes to apply the modulation controlling electric field. Those RF electrodes are not part of any RF waveguide. Those “bulk” electrodes typically are connected to an RF signal source by means of an RF cable and wires and represent the termination point for the RF cable. In contrast to these prior art devices, the RF electrodes in some embodiments of the present invention are part of a transverse-electromagnetic (TEM) RF waveguide or part of a mircostrip-transmission line RF waveguide. The RF field propagates along the RF waveguide.
Optical modulators that comprise a cascade of multiple gratings separated by optical-waveguide sections are described in articles by Shaw et al. (Electronics Letters, vol. 35, no. 18, 2 Sep. 1999, pp. 1557-1558), by Taylor (Journal of Lightwave Technology, vol. 17, no. 10, October 1999, pp. 1875-1883) and by Khurgin et al. (Optics Letters, vol. 25, 2000, pp. 70-72). FIG. 2 shows an illustration of these prior modulators. In these modulators, the RF electrodes are a part of a traveling-wave RF waveguide. The traveling RF wave propagates in this RF waveguide in the same direction as does the light carried in the optical waveguide. The function of the multiple gratings is to serve as optical reflectors, with each pair of such grating reflectors and the optical waveguide segment between them acting as an optical etalon. The cascade of etalons slows down the group velocity of the light propagating through that cascade. A goal of these prior devices is to match the velocity of the traveling RF field with the velocity of the traveling optical field (the light being modulated). In contrast to these prior art devices, the RF field of the present disclosure travels in a direction that is substantially perpendicular to the direction of travel of the light being modulated. The issue of velocity matching, the goal of these prior devices, is obviated by the present disclosure.
The prior art modulators have involved either single gratings with bulk electrodes or, when they have involved multiple gratings with traveling-wave electrodes, the RF field in those modulators travels in the same direction as the optical field. In contrast, for the multiple-grating modulators of the present invention, the RF field travels, in an RF waveguide or transmission line, in a direction that is essentially perpendicular to the direction in which the optical field travels.
The constraint in the present application about the maximum length of a given grating involves known principles concerning time-varying modulation and could be considered as known in the prior art. However, the constraint in the present disclosure about the length of the waveguide segment between two gratings is a result of the unique use of orthogonal propagation directions for the RF and optical fields. Such a constraint would have no relevance to any of the aforementioned prior art modulators.